The present invention relates to a vacuum holddown device for hardcopy apparatus and in particular to a vacuum mechanism for a printer which permits a change between a higher vacuum during a print phase and a lower vacuum or no vacuum during a print media advance phase.
The vacuum in the printzone of a printer needs to be applied during printing to hold the print media in the correct position and at the correct spacing under the printhead. This also serves to reduce expansion of the print media as it absorbs ink (i.e. cockle) and ensures a good print quality. On the other hand, the vacuum needs to be removed or substantially reduced while the print media is being advanced in order to reduce friction and to keep the required media positioning accuracy.
During the time the vacuum is rising and falling between its high and low values, the printer is idle, since the vacuum is too high to advance the media, but too low to permit satisfactory printing. Thus the problem arises of how to switch the vacuum in the printzone on and off in the minimum possible time so that it does not adversely affect printing time and thus the throughput of the printer.
In one previous proposal, a fan is used to produce a vacuum in a chamber below a printzone with a plurality of holes in a printing platen between the chamber and the print media. The vacuum pressure can be changed by switching the fan off and on to pressurise and depressurise the chamber. However, this has the disadvantage of taking a long time to change between maximum and minimum pressure levels, typically of the order of tenths of seconds.
In another previous proposal, two vacuum accumulation chambers are provided, one at a high level and the other at a low vacuum level, and a valve is provided to switch the connection of the main vacuum chamber under the printzone to one or the other of the accumulation chambers. The time taken to pressurise and depressurise the main vacuum chamber depends on the relative size of the chambers and also the power used to generate the vacuum. An efficient arrangement has a high power requirement and the volume of each of the vacuum accumulation chambers needs to be at least one order of magnitude larger than that of the main vacuum chamber, leading to high space requirements.
Certain aspects of the present invention seek to overcome or reduce one or more of the above problems.
The present invention seeks to provide a vacuum holddown arrangement which is simple, compact, efficient and inexpensive.
According to a first aspect of the present invention there is provided a vacuum holddown device for a hardcopy apparatus comprising a printing platen, said printing platen having holes therethrough, said holes being arranged in a predetermined pattern, a vacuum chamber, and a moveable member, said moveable member including a planar portion, said planar portion having holes therethrough arranged in substantially the same predetermined pattern, and said moveable member being moveable between a first position, in which said holes in said moveable member are substantially in alignment with said holes in said printing platen and said holes in said printing platen are in communication with said vacuum chamber, and a second position, in which said holes in said moveable member are out of alignment with said holes in said printing platen and said holes in said printing platen are not in communication with said vacuum chamber.
According to a second aspect of the present invention there is provided a vacuum holddown device for hardcopy apparatus comprising a printing platen having a set of through holes in a predetermined pattern, a moveable member having a set of through holes in substantially the same pattern, and a vacuum chamber, the moveable member being located between said printing platen and said vacuum chamber.
According to a third aspect of the present invention there is provided a method of applying two different pressure levels to a printing platen of a vacuum holddown device of a hardcopy apparatus, said platen having a first set of through holes located to be in communication with a vacuum chamber, the method comprising providing a moveable member between said platen and said vacuum chamber, said moveable member having a second set of through holes, the holes in said first and second sets being arranged in substantially the same pattern, and the method further comprising moving said moveable member between a first position and a second position, said holes in said two sets being in substantial alignment in said first position wherein said first set of holes is in communication with said vacuum chamber, and said holes in said two sets being out of alignment in said second position wherein said first set of holes is not in communication with said vacuum chamber.